This invention is a process for treating raw water and producing potable water meeting accepted purity standards.
Surface water from rivers, lakes or reservoirs is treated in a water plant to convert the surface water into water for human consumption meeting accepted purity standards. The processes used have been honed over the years to produce high quality potable water. Surface water which has not been chemically treated is referred to by the industry as raw water and is referenced as same herein.
The process for treating water includes passing raw water through the municipal water plant while treating the raw water as it passes through the water plant. Primary treatment to the raw water in a water plant occurs in a clarifier or a basin (also referred to as a settling basin) after a flocculant has been added to raw water. The flocculant causes particles suspended in the water to coagulate, subsequently growing in size and weight. A water plant clarifier is a large sized, usually round structure whereas a basin is usually rectangular. The flocculant is generally added to the raw water in a small mixing chamber referred to as the rapid mix or flash mixer, to facilitate thorough, uniform mixing with the raw water. Depending on the design of the water plant, this chamber may be placed or positioned in various locations. In particular, the mixing chamber may be provided within the clarifier such as in a centerwell clarifier or adjacent to a basin. In other water plant designs, the mixing chamber may be provided upstream of the clarifier or the basin. The clarifier or the basin is generally sized to provide sufficient residence time for the majority of the solids to drop out of suspension. When necessary, the flocculant may be added to the water being treated in the water plant anywhere along the transfer pipe which delivers raw water from the river, lake or reservoir to the water plant to increase the total reaction time for the flocculant. Water is then passed through sand filters, perhaps treated with activated carbon, chlorinated and possibly fluoridated before being delivered to water supply mains which transport the treated water to residences, businesses and industries.
One of the recurrent problems in water treatment plant operations is the growth of algae in the clarifier or in the basin and sand filters. Algae come in many types including filamentous algae, such as Cladaphora and Spirogyra, planktonic algae such as Microcystis and Anabaena, branched algae such as Chara vulgaris and Nitellam, swimming pool algae commonly referred to as black, brown and red algae and algae found in ponds such as Dictyosphaerium, Spirogyra, Oedogonium, Chlorococcum, Pithophora, Hyudrodictyon and Lyngbya. It is not uncommon to see a municipal water plant clarifier or basin with a beard of algae around its peripheral walls and filamentous algae growths several feet long.
As used herein, the term municipal water plant is intended to mean a water plant used in treating raw water and converting it to potable water for human consumption, regardless of whether the entity doing so is public or private.
Algae blooms have been noted to appear literally overnight under the right temperature and sunlight conditions and, if left untreated, will cause taste and odor problems in the finished waters. The problems caused by algae in municipal water plants are handled in a variety of ways by current treatment methods. The taste and odor problems which typically recur during periods of high summer temperatures and long daylight hours occur from detritus thrown off by algae in the clarifier or basin. Not all of this detritus is removed by sand filters. A portion of the detritus passing through the sand filters is converted in the final chlorination process to a family of chloro-organic compounds which contribute to the objectionable taste and smell that consumers complain about.
The standard treatment for controlling algae in municipal water plants is to scatter crystals of cupric sulfate pentahydrate, CuSO4.5H2O, which is also known by its common name blue vitriol, into the water. Blue vitriol is commercially available in 50 pound bags having crystals ranging in size from fine (xe2x85x9xe2x80x3) to large (1xe2x80x3). Scattering is done with a shovel, a scoop, or by hand. Ideally, the crystals dissolve in the water so the copper ion is present in the water. The soluble or active copper (II) ion kills algae because of its effect on chlorophyll which is a large porphyrin structure occurring either as blue-green chlorophyll-a or yellow-green chlorophyll-b. Both molecules have four centrally placed nitrogen atoms which complex a single magnesium atom. The magnesium removes carbon dioxide from the water and delivers it to the algae thus allowing photosynthetic growth. The soluble copper (II) ion replaces the magnesium by forming a stronger porphyrin complex, which does not bond with carbon dioxide. The algae die by virtue of its growth mechanism being squelched by a lack of carbon dioxide, in a process analogous to the chemical poisoning of hemoglobin in mammals. One of the inherent advantages of copper algicides is that algae cannot mutate or evolve to avoid its effect. No amount of evolution can prevent copper from displacing magnesium in the chlorophyll and no amount of evolution can cause the copper porphyrin to absorb carbon dioxide.
Disclosures of some interest are found in U.S. Pat. Nos. 3,844,760; 4,012,221; 4,505,734 and 5,541,150.
The above description of the prior art is an idealized situation but which has a number of practical problems and disadvantages, some subtle and some not so subtle. A substantial part of the blue vitriol does not dissolve because it is difficult to dissolve in water which is not acidic. Plainly put, blue vitriol crystals do not dissolve very well in pH 7, or more alkaline water. Since the incoming raw water most often has a pH of 7 or above, this causes the crystals to acquire a coating of copper hydroxide that inhibits dissolution of the blue vitriol crystals into the water. Thus, much of the copper sulfate is wasted and ends up in the settled sludge in its undissolved form. Consequently, only a small portion is consumed, as intended, by intimate bonding to the algae chlorophyll. In addition, scattering blue vitriol crystals does not produce uniform dosages of copper sulfate in the water. Instead, very high dosages will be found immediately down current from the crystals and little copper sulfate will be found elsewhere.
In this invention, a water soluble copper salt is dissolved in an aqueous acidified solution such as an acidic flocculant because many water soluble copper salts, and the preferred copper sulfate, are much more soluble in low pH water than in neutral to high pH water. The resultant algicide-flocculant solution in accordance with the present method can be delivered in a tank truck or by a tank rail car and off loaded into storage tanks.
In the manner previously described, the treating of raw water in accordance with the present invention includes the standard steps of passing raw water through the municipal water plant while treating the raw water as it passes through the municipal water plant. In this regard, it should be appreciated that water continually flows through the various parts of the water plant such as the clarifier and/or basin as it is being treated in the water plant. In accordance with the present invention, the algicide-flocculant solution as described in further detail below is metered into the raw water being treated at an injection point which allows thorough mixing of the algicide-flocculant solution with the raw water as it continually flows through the water plant and is treated therein. In this regard, the solution may be metered into the raw water as it passes through the clarifier via a mixing chamber in the centerwell, or as it passes through the basin via a mixing chamber adjacent to the basin. In other applications, the algicide-flocculant solution may be metered into the raw water somewhere upstream of the clarifier and/or the basin via a mixing chamber located upstream of the clarifier and/or the basin. Such a mixing chamber may include a rapid mix or a flash mixer both known in the art which facilitates thorough uniform mixing of the flocculant and copper algicide with the raw water being treated at the water plant. Of course, other methods or devices may also be used. Metering pumps are generally designed to deliver a predetermined, controlled amount of a liquid and their use enables delivery of the copper algicide with the flocculant in a simple and efficient manner. The turbulence of the water stream also provides thorough and uniform mixing of the algicide with the water, as contrasted to the prior art technique of scattering blue vitriol crystals. Efficient mixing of the copper algicide with the water provides low, uniform dosages of copper which is very desirable because little copper is wasted.
The copper solution provides copper (II) ions that displace the magnesium ion in chlorophyll to kill the algae in the clarifier or basin. The amount of copper in the algicide-flocculant solution is controlled; thus the amount of copper added to the raw water is also controlled and is maintained at low levels. The copper reacts with the magnesium in the chlorophyll molecules and, along with the dead algae, collects in the sludge in the bottom of the clarifier and/or the basin.
When using this invention, no blue copper crystals will be found in the settled sludge, which means that more of the copper has been put to its intended use of killing algae rather than being wasted. In addition, the amount of soluble copper ion passing through the clarifier or basin into the finished water will normally not exceed 0.1 ppm which is well below the 1.3 ppm standard required by the Lead and Copper Rule of the Environmental Protection Agency.
It is difficult to overstate the importance of low, uniform dosages of copper. For the algicide to be effective, copper (II) ions must come intimately close to the magnesium ion in the chlorophyll complex of substantially all of the algae cells. This can be accomplished with improved mixing and distribution of the algicide when it is combined with the flocculant as in this invention so the algicide-flocculant solution then being mixed with the water being treated at the water plant provides uniform dosage of the copper. Uniform dosages are the key to effectiveness while low concentrations reduce treatment costs.
In the past, a water plant has typically used a conventional flocculant, either with or without a polymeric flocculant aid. With the onset of a substantial algae bloom, attempts would be made to control the algae bloom using the prior art technique with less than satisfactory results. When facing a full grown algae bloom, the amount of copper in the algicide-flocculant solution of this invention would be at a relatively high level which will bring the algae bloom under control in a fairly short period. After the algae bloom is brought under control, the amount of soluble algicide will be reduced in subsequent batches of algicide-flocculant solutions and ultimately reduced to a lower level that is sufficient to keep algae growth suppressed. As will be appreciated by one of ordinary skill in the art, the subsequent batches of algicide-flocculant solution is not generally added to the same body of water already treated with the initial batch of the algicide-flocculant solution since water is continually flowing through the water plant as it is being treated in a continuous treatment process. Because the algae bloom is brought under control with the algicide-flocculant solution having the higher initial dosage of copper, the subsequent batches can have reduced dosages of copper to maintain this control as the water is continually processed through the water plant. A large proportion of the algicidal copper exits the treated water stream in the settled sludge and not with the finished water because it has been intimately bonded to the algae chlorophyll. The water is then passed through filters such as sand filters. In addition, the water may be further treated with activated carbon, chlorinated and possibly fluoridated before being delivered to water supply mains which transport the treated water to residences, businesses and industries.
An object of this invention is to provide an improved technique for treating algae in a municipal water plant.
A more specific object of this invention is to treat raw water with an algicide-flocculant solution which, drops particulates out of suspension and simultaneously controls algae in the clarifier, the basin, and/or the sand filters.
These and other objects and advantages of this invention will become more apparent as this description proceeds, reference being made to the appended claims.
The copper algicide of this invention is selected from water soluble copper salts. From a simple algicidal standpoint, almost any water soluble copper salt is suitable. From the standpoint of producing potable water, the choice is more limited because not all water soluble copper salts can economically be put into drinking water. Thus, the common choices for the water soluble copper salt are copper sulfate, copper chloride, copper nitrate and copper acetate. The selection will likely be based on the relative cost of copper salts. Copper sulfate is the preferred water soluble copper salt because it is the only one presently approved for use in municipal water plants; it is effective as a source of copper (II) ions; and, it is the least expensive of the possible candidates. The preferred form of copper sulfate is blue vitriol which is cupric sulfate pentahydrate.
The amount of blue vitriol in the algicide-flocculant solution varies between 0.1-5% by weight. The proportion of copper in blue vitriol is 25.45% by weight which means that the copper concentration in the algicide-flocculant solution varies from about 0.025-1.275% by weight. Preferably, the amount of blue vitriol in the algicide-flocculant solution is 0.1-1% by weight meaning that the active copper concentration in the preferred solution is about 0.025-0.25% by weight. The equivalent concentration of other soluble copper salts is found in Table I:
Accordingly, the concentration of the copper salts vary from about 0.05% to about 6% by weight in order to provide the desired range of active copper concentration.
As noted above, the copper concentration in the algicide-flocculant solution in accordance with the present invention is between 0.025-1.275 wt %. It has been found in one preferred embodiment, that an active copper concentration of 0.25-0.765 wt % in the algicide-flocculant solution may be needed to bring algae blooms under control.
As also previously described, in accordance with one embodiment of the present invention, the water may be treated by adding the algicide-flocculant solution in a series of batches preferably with progressively reduced copper concentration to effectively control the algae bloom. In this regard, initial batches of algicide-flocculant solution would have relatively high copper concentration such as approximately 0.38-1.275 wt %, or preferably 0.25-0.765 wt % for example, so that the algae die off substantially. Then, subsequent batches of algicide-flocculant solution preferably containing reduced amounts of active copper algicide are provided to further reduce the algae levels.
In this regard, the subsequent batches of algicide-flocculant solution may be about half the initial dose. Copper concentrations in the algicide-flocculant solution in the range of about 0.025-0.38 wt %, or preferably, 0.125-0.38 wt % are usually sufficient as a maintenance dose to keep algae under control and prevent the formation of algae blooms, even under the most trying conditions of temperature and sunlight. For instance, a second batch of the algicide-flocculant solution may have a copper concentration of less than about 0.38 wt % such as 0.125-0.38 wt %, and a third batch of the algicide-flocculant solution may have a copper concentration of about 0.125-0.25 wt %. As previously described, the subsequent batches of algicide-flocculant solution are not generally added to the same body of water already treated with the initial batch of the algicide-flocculant solution since water is continually flowing through the water plant or parts thereof such as the clarifier and/or the basin as it is being treated in the water plant but is added to the raw water continually flowing into the water plant. Again, the subsequent batches preferably have a reduced dosage of copper to maintain this control as the water is continually processed through the water plant.
Moreover, as previously noted, the algicide-flocculant solution may be metered into the raw water as it passes through the clarifier via a mixing chamber in the centerwell, or as it passes through the basin via a mixing chamber adjacent to the basin. In other applications, the algicide-flocculant solution may be metered into the raw water somewhere upstream of the clarifier and/or the basin via a mixing chamber located upstream of the clarifier and/or the basin. The water is then passed through filters such as sand or other appropriate media. In addition, the water may be further treated with activated carbon, chlorinated and possibly fluoridated before being delivered to water supply mains which transport the treated water to residences, businesses and industries. Because the water has been effectively treated and the algae controlled, the taste and odor problems associated with detritus thrown off by algae in the clarifier or basin are minimized.
The specific location or part of the water plant where the algicide-flocculant solution is metered into the water as it flows through and is treated in the water plant largely depends on the design and configuration of the water plant to which the present invention is applied. In this regard, it should be appreciated that in accordance with the present method, the algicide-flocculant solution as described in further detail herein below can be added to the raw water being treated at any appropriate location or part of the water plant and the above specific locations are merely provided to suggest locations for some commonly used water plant designs and configurations. However, it should further be noted that significant advantages are provided by adding the algicide-flocculant solution to the clarifier or upstream of the clarifier since algae growth can then be readily controlled downstream of the location at which the algicide-flocculant solution is added. For instance, by adding the algicide-flocculant solution to a mixing chamber such as a rapid mix located upstream of the clarifier, algae growth in the clarifier and/or the basin can be readily controlled. In addition, by adding the algicide-flocculant solution upstream of the clarifier and/or the basing, the algicide-flocculant solution would allow control of algae growth in the filter and any other part of the water plant further down stream in the water treatment process.
In order to make the algicide-flocculant solution as described, it is theoretically possible to add copper salt crystals in the proper proportion to an acidic flocculant solution and agitate the solution to dissolve the copper salt. In practice, this has not been efficient for a variety of reasons. The source of blue vitriol is crystals which require vigorous agitation to dissolve, such as occurs with a powered impeller. With aluminum sulfate as the flocculant, solubility is adversely affected by the common ion effect. It is accordingly much better to dissolve the copper salt in water and then mix the water soluble copper salt solution with the flocculant solution. Using copper sulfate as the algicide, one part blue vitriol is dissolved in two parts water making a nearly saturated copper sulfate solution.
In accordance with one embodiment, to prepare the water solution of the copper salt, a mixing tank is partially filled with water and a suitable mixer, such as a powered impeller, is used to agitate the water. Preferably, the water is heated with a suitable heater, such as an electrically powered immersion heater or preheated by conventional methods such as a water heater. The selected copper salt is taken from commercially available bags and the desired quantity added to the tank. Using blue vitriol, the water solution will initially be bluish but somewhat milky which is caused by partial formation of copper hydroxide. Continued stirring and complete dissolving of the blue vitriol will result in a clear blue color typical of copper sulfate solutions.
During the mixing process in accordance with one embodiment, the copper salt solution is acidified to a pH of no more than 5 and preferably in the range of 4-5. This may be accomplished by adding a small quantity of acidic flocculant solution into the tank, typical flocculent solution having a pH of about 2.5, which is about the same as lemon juice. Acidifying the solution prevents the formation of copper hydroxide so the copper salt completely dissolves and remains in solution. Acidifying the solution with the flocculant material is advantageous in that it avoids using a different acid material which, in the treatment of water for human consumption, might provide regulatory problems.
Suitable flocculants of this invention are aluminum sulfate, iron sulfate, iron chloride and mixtures thereof. Polymeric aluminum flocculants such as aluminum chlorohydrate, polyaluminum chloride, polyaluminum sulfate, and mixtures thereof may also be used. Preferably, but not necessarily, the flocculants are prepared in a nearly saturated solution. In a typical process, aluminum oxide is reacted with sulfuric acid to produce liquid aluminum sulfate, i.e. about 47-50% by weight aluminum sulfate in water. Iron sulfate, iron chloride, and the polymeric aluminum flocculants may be prepared by commonly known procedures, as is well known in the art. In this invention, the amount of flocculant in the algicide-flocculant solution varies between 25-50% by weight and preferably is 35-50% by weight.
The invention is also useable with polymer flocculant aids of any suitable type. Polymer flocculant aids are long chain, high molecular weight cationic materials, usually having molecular weights in the range of 20,000-800,000. Conventional flocculants, such as aluminum sulfate, iron sulfate, iron chloride, aluminum chlorohydrate and mixtures thereof, produce relatively small flocs which require relatively long residence times to settle out by gravity in the clarifier and/or the basin. The polymer flocculent aids cause these small flocs to agglomerate into larger particles that settle at faster rates, thereby allowing shorter residence times in the clarifier and/or the basin. The present standard polymer flocculent aids are high molecular weight quaternary amines such as diallyldimethylammonium chloride or dimethylamine epichlorohydrin which are commercially available from various domestic manufacturers such as Ciba Specialty Chemicals (previously known as CPS Chemical Company) of West Memphis, Ark. In this invention, the amount of polymer flocculant aid in the algicide-flocculant solution varies between 0-10% by weight but preferably is 0-5% by weight. In one embodiment, the polymer flocculent aid may be added to the flocculant prior to the addition of the water soluble copper salt solution.
Potable water treatment chemicals are typically delivered by tank truck to the water plant. In this invention, the flocculent, with or without the cationic polymer aid, and the acidified copper salt solution are thoroughly mixed in a processing vessel or tank by agitation, air mixing or a recirculating pump. The complete homogenous mixture is then loaded into the tank truck or trailer for delivery to the water plant. In an alternate production method, the flocculant, with or without the cationic polymer aid, is simultaneously loaded with the acidified copper salt solution into the tank of a tank truck or a trailer. Final mixing occurs during transport, caused by agitation of the liquid contents due to movement of the truck/trailer.
The algicide-flocculant solution, with or without the polymeric flocculant aid, is added to the raw water using conventional metering equipment to deliver sufficient flocculent to coagulate the particulates in the raw water. As previously described, the algicide-flocculant solution as described can be metered into the raw water via the clarifier, the basin, or via a mixing chamber provided upstream of the clarifier and/or the basin, or any other appropriate location in the water plant to control algae growth.
In accordance with one embodiment, nearly saturated flocculant is added to the raw water in the range of 20-60 ppm, an average value being about 30 ppm. Because incoming raw water contains very little soluble copper, the active copper concentration in the clarifier and/or the basin is due almost entirely to the copper algicide combined with the flocculant. Thus, the treated water may have a copper concentration in the range of approximately 0.025-0.764 ppm.
Examples of how the algicide-flocculant solution can be prepared and/or used in accordance with the present invention to treat water include: